As will be well understood by those skilled in the art, congestion occurs when a large number of users/devices contend for limited resources.
For instance, in the plain old telephony (POT) domain, a plurality of users may use a private branch exchange (PBX) with a limited number of trunks. As a result, when more users than the available number of trunks attempt to place an outbound call at the same time some of the users will not be able to obtain an outside line, thus resulting in being prevented from placing the outbound call. For example, assume a company with 1000 employees has a PBX with 100 trunk lines connecting the PBX to a central office. When the first 100 employees attempt to place an outside call at approximately the same time, they would be able to get an outside line and place outgoing calls. If another employee attempts to place an outbound call at the same time when the 100 trunk lines are used by the first 100 callers, that call would fail and the user may hear a busy tone, a turkey tone, or a specific message advising him that all outside lines are busy. The ongoing calls of the first 100 users/employees would continue uninterrupted. In this case each user uses a dedicated resource which is controlled by a central call processing program which has visibility to the utilization of each resource; new users are prevented from accessing resources which are used by other callers thus allowing the first users to continue using the resource without interruption.
In computer networks (e.g., Internet Protocol or “IP” networks), traffic from various users traverses a common media which has limited capacity. To ensure communication integrity only one user can utilize the common media at the same time requiring interleaving in time messages from different users. Unlike the PBX scenario where a central call processing software controls the access of the phones (terminals) to the common resources (trunk lines), IP networks generally operate without central control. In order to reduce potential collisions between transmissions from different terminals over the common media, the carrier sense multiple access with collision detection (CSMA/CD) control method was developed. The method uses a carrier sensing scheme in which a transmitting station listens on the common carrier before attempting to transmit. If it senses that another terminal is transmitting, it backs off for a random period of time and refrains from transmitting before making another attempt to transmit. In case two (or more) stations collide (due to critical race) the terminals detect the collision, stop transmitting, and wait for a random time interval before trying to transmit again. The CSMA/CD takes advantage of two key elements: a) terminals can sense transmission from any other terminal with which they share the common media, and b) terminals share a short media over which signals traverse almost at the speed of light. Therefore the probability that a terminal starts transmitting at the same time that another terminal starts transmitting (because the two terminals have not sensed the carrier signal from the other terminal) is extremely low.
In satellite communications, on the other hand, multiple mobile users/devices may attempt to exchange messages simultaneously over a shared channel. This is termed multiple access, and may be accomplished with spread-spectrum multiple access techniques, such as code division multiple access (CDMA). Congestion mitigation techniques such as time division multiple access (TDMA) or frequency division multiple access (FDMA) are not always available in satellite communications. In particular, for TDMA, a system with disparate users without a common clock source does not allow synchronization between independent transmitters. Conversely, FDMA requires available frequencies, but many satellite-based communication systems are configured to use all available bandwidth to support more and more users, often limiting or even eliminating the use of FDMA. Further, for a variety of reasons, mobile terminals in a satellite communication network often cannot sense transmission from other terminals. For example, transmission power is directed away from the earth and not detectable by other users that can be entire continent away. Moreover, in satellite communications, the medium that is shared may be a geostationary satellite link which operates over extremely long distances and therefore has hundreds of milliseconds of delay between transmitter and ground station in each direction.
In particular, low power mobile devices or terminals with small antenna apertures, and thus low equivalent isotropically radiated power (EIRP), may attempt to communicate via geostationary satellites. Such a system will generally result in very low received signal power. Additionally, a receiver at the satellite ground station which attempts to receive a signal from a given terminal (via a satellite) may incur further degradation in its signal-to-noise ratios (SNRs) due to signals from other terminals. The signal from other terminals (conveyed via the satellite) may cause co-channel interference (CCI) when other emitters, such as other terminals, transmit at the same time at the same frequency band. In general, as the number of co-transmitters increase, the noise floor at the intended ground station receiver increases resulting in reduced SNR at the receiver. As a result of this CCI, a ground station receiver associated with a specific satellite may be able to support only a limited number of simultaneous terminal transmissions before CCI increases the noise floor to a level that prevents the receiver from being able to decode incoming signals from any terminals rendering the communication channel blocked. If the number of simultaneous emitters exceeds that nominal number, the receiver may fail to properly decode the transmissions from any and/or all mobile devices.
Another limiting condition, other than CCI, is that such communications may be subject to regulatory power spectral density restrictions which may prevent increasing the transmission power of a given terminal. In a specific case of the regulatory restriction, a regulatory specification may place a limit on the number of simultaneous terminal emitters (in a specific frequency band) to prevent transmitters from adversely affecting other users in a given system. If the number of simultaneous emitters exceeds that limit, the associated communication license will be violated, and the operator may be subject to severe penalties. As described above, mobile uncoordinated terminals communicating over satellite links cannot sense transmission by other geographically dispersed mobile devices and as such have no indication that they should refrain from transmission when the regulatory limit is reached.
Still further, in order to ensure proper communication, transmitted messages are typically acknowledged by the receiver. When a transmitting mobile device does not receive a returned acknowledgement from the receiver within a predefined time period, the mobile device may attempt to resend the original message. As such, the attempted retransmission may contribute to even greater congestion at the receiver, and if the number of received transmissions at the receiver exceeds its capacity, it may result in failed reception again. Such failed reception would invoke yet another retransmit attempt that could bring the network down in a process similar to a denial of service (DoS) attack. Unlike the IP network scenario, uncoordinated mobile terminals communicating over satellite links cannot sense transmissions by other geographically dispersed mobile devices, and as such have no indication that they should refrain from transmitting when the communication channel is fully utilized and cannot accommodate another transmission session.